Think Scrabble with numbers and you'll have a pretty good idea how the game of Sumoku works.
Each student or pair takes 8 number tiles and, without using any colour more than once in a single 'word', works out how to achieve the most points by adding the single digits of their 'word' together. There's a catch, of course. To start the game you roll a special die to decide what multiples are allowed, so each 'word' must be a multiple of the magic number. The choices are multiples of 3, 4 or 5.
It's a popular activity that can be played with or without keeping score, depending on the abilities and personalities of the students playing.